Patent GB-998948-A (GB)
Title: Content addressable memory
IPC: G11C15/02
Filed: 1964-03-26
Abstract: 998,948. Tunnel diode pulse circuits; circuits employing bi-stable magneticelements. BUNKERRAMO CORPORATION. March 26, 1964 [March 29, 1963], No. 13009/64. Headings H3B and H3T. [Also in Division G4] In a content addressable memory storing a plurality of data words in respective word lines, the digit lines are interrogated sequentially, the resulting trains of output pulses on the word lines being employed to determine the equality relationship or relative magnitude between the interrogation word and the various stored words. The memory described, Fig. 1, stores N 5-bit words, the bi-stable, non-destructive read-out, storage elements 16 (each called a " Biax ") each comprising a block of magnetic material (Fig. 2(a), not shown) having perpendicularly bored apertures for the word line and digit line, the read-out currents on the word line consisting of a pulse for each change in the input current on the digit line, the polarity of a pulse for a given directional digit line current change depending on the value of the stored bit, see Fig. 2 (b). Equality comparison.-The interrogation word, assumed to be 01001, is stored in a flipflop register 12. In operation, a control counter 40 produces timing pulses T 0 -T 6 , the pulses T 1 -T 5  being applied in turn to the five stages of the register 12, each pulse being applied to both sides of the corresponding flipflop which is thereby changed over regardless of its original state, the consequent potentials on the digit lines of the memory being shown in Fig. 4(b)-(f). The output waveforms on word lines I-N are shown in Fig. 4(g)-(l) equality between the interrogation word and a stored word being indicated by the production of negative pulses only, Fig. 4(k). Relative magnitude comparison.-Although a relative magnitude comparison can be made directly from an examination the waveform of Fig. 4(g) - (l) it is generally convenient to provide a sensing device such as that shown in Fig. 3(a) which comprises a pair of magnetic cores 50, 52 connected as shown for each word line W 1 -W N . To determine which stored words are less than or equal to the interrogation word the cores 50, 52 are all initially switched to the state shown by the arrows and the waveform of Fig. 4(n) is then applied to the " inhibit " line passing through the cores 52, the pulses of the waveform corresponding to "1's" in the interrogation word so that the cores 52 are inhibited from switching at these times. The waveforms, Fig. 4 (g)-(l) produced during the comparison operation are received on line WI-W K , Fig. 3(a), the diodes 58 allowing the first positive pulse received on each row line W 1 -W N  to switch the corresponding core 50 thereby producing an output on connecting link 60 to switch the corresponding core 52 only if there is no pulse on the inhibit line. At the conclusion of the operation, there remain in their original state only those cores 52 corresponding to a successful comparison operation. To perform on " equal to or greater than " comparison, the waveform of Fig, 4(p) is applied to the inhibit line instead of the waveform of Fig. 4(n). The cores 52 may be scanned sequentially to determine their state by a circuit 66. Tunnel diode circuit.-The magnetic core sensing device of Fig. 3(a) may be replaced by the tunnel diode circuits shown in Fig. 3(b). Each bi-stable circuit comprises a tunnel diode 104 set initially to its low voltage state by closing a switch 107. The train of positive and negative pulses resulting from the comparison operation is applied to the line W1, the first positive pulse switching the tunnel diode 104 to its high voltage state in which it remains until the end of the operation. The switching of tunnel diode 104 couples a positive pulse via a capacitor 106 to the base of a transistor 108 thereby causing a collector-emitter current which passes through a first primary 109 of a transformer 110, but only if no current is applied to a second primary 118 of the transformer, is a current induced in the secondary 120 to switch a bi-stable element 122.